This invention relates to the domain of radiocommunication systems with mobiles.
More precisely, the invention relates to mobile radiocommunication terminals used in this type of system.
The invention is applicable to any mobile radiocommunication terminal designed to receive and possibly emit signals, and particularly but not exclusively for radiocommunication systems of the GSM 900 (Global System for Mobile operating within the 900 MHz band) type, or the DCS 1800 (Digital Cellular System 1800 MHz) type, or the PCS 1900 (Personal Communication System 1900 MHz) type, or the UMTS (Universal Mobile Telecommunication System 2 GHz) type, or the CDMA (Coded Division Multiple Access) type.
Note that a mobile radiocommunication terminal is physical equipment used by a user of a radiocommunication system network to access telecommunications services offered through a base station.
Note also that a base station covers a given geographic area (or cell) in which many mobile radiocommunication terminals can move, and with which the base station can exchange signals.
Depending on the system, the mobile radiocommunication terminal may be called a mobile station, a portable telephone, a mobile radiotelephone, or a radiocommunication terminal. The expression xe2x80x9cmobile terminalxe2x80x9d will be used throughout the remainder of this patent document, for simplification purposes.
Conventionally, a mobile terminal comprises at least one antenna designed to receive and/or transmit signals from and/or to a base station.
Also conventionally, a xe2x80x9cwhipxe2x80x9d type antenna with a linear polarization is used.
In general, it is observed that signals exchanged between the mobile terminal and the base station are subject to fading and/or echoes and/or interference that can generate partial or total loss of the transported information.
Note that:
fading phenomena occur particularly when two signals with similar powers and opposite phases reach the same reception antenna at the same time, the resulting signal therefore being practically zero;
echo phenomena are generated particularly by transmitted signals that xe2x80x9creflectxe2x80x9d on obstacles encountered on the path between a mobile terminal and the corresponding base station;
interference phenomena are provoked particularly when signals with similar characteristics (such as frequency) are being carried in the same propagation medium.
In other words, exchanged signals are subject to disturbances caused particularly by propagation conditions (sometimes difficult) in the transmission medium. Therefore, it is easy to understand that this type of disturbance can generate crucial problems concerning the quality of signals transmitted by a base station and retrieved by a mobile terminal.
Thus, it is sometimes difficult to use signals received by the mobile terminal.
Furthermore, the position of the antenna on the mobile terminal is such that its radiation is partly absorbed particularly by the user""s head, when the user keeps the mobile terminal close to his ears. Consequently, this proximity between the mobile terminal and the user""s head partially reduces the energy received and emitted by the mobile terminal.
Finally, the reception quality of the mobile terminal usually depends on the position of the mobile terminal and/or its orientation with respect to the base station.
The purpose of this invention is to overcome these various disadvantages with the state of the art.
More precisely, one of the purposes of this invention is to provide a mobile terminal with good reception quality even under difficult conditions (and particularly in the presence of fading phenomena and/or interference and/or echoes).
Another purpose of the invention is to provide this type of mobile terminal capable of obtaining reception practically independent of the position and/or orientation of the mobile terminal with respect to the base station.
Another purpose is to provide this type of mobile terminal that does not modify exchanged signals while maintaining a good reception quality.
These various purposes, and others which will become clearer later, are achieved according to the invention by means of a mobile radiocommunication terminal of the type used in a radiocommunication system, characterized in that it comprises:
at least one first antenna, used in reception and with at least a first polarization;
at least one second antenna, used in reception and with at least one second polarization,
separate from the first polarization; and
means of combining the received signals, in polarization diversity, by the said first and second antennas.
Therefore the general principle of this invention is based on the association of several distinct polarizations used in reception mode.
It will be understood that the diversity of polarizations makes it possible to offer several distinct reception channels, namely one or more for the first polarization(s) and one or more for the second polarization(s).
Thus, a combination of various polarizations increases the probability of correct reception of signals emitted by the base station. In particular, this results in a gain of a few dB in the reception sensitivity. Consequently, the increased probability of signal reception makes the mobile terminal much less sensitive to fading and/or interference and/or echo phenomena.
Note that the reduction in the influence particularly of fading phenomena on signal reception enables better performance of power control algorithms (Rayleigh or Rice process).
Consequently, the mobile terminal according to the invention can work under difficult conditions for propagation of signals in the transmission medium.
Note that the different reception channels with distinct polarizations are preferably independent and decorrelated from each other, in order to optimize the manner in which the mobile terminal processes received signals. It is obvious that an expert in the subject would know how to use the different antennas such that the different reception channels generated are independent and decorrelated from each other.
The means of combining the received signals are the means that make it possible to optimally process signals received from the various possible reception channels, in order to rebuild the signals transmitted by the base station as faithfully as possible.
Advantageously, the said first polarization belongs to the group comprising rectilinear polarizations.
Preferably, the said second polarization belongs to the group comprising:
circular polarizations,
elliptical polarizations.
Note that the fact that there is at least one circular polarization in association with at least one linear polarization enables the mobile terminal to receive signals intended for it independently of its position and/or orientation with respect to the base station.
Advantageously, the said first antenna(s) belong(s) to the group comprising:
whip antennas;
panel antennas.
Obviously, this list is not restrictive.
Preferably, the said second antenna(s) belong(s) to the group comprising panel antennas.
This list is not exhaustive.
Preferably, the said antenna(s) is (are) used with a double polarization, right and left.
In this way, a total of three separate polarizations (two circular and one linear) is obtained. The use of this type of double circular polarization contributes to reducing the influence of fading and/or interference and/or echo phenomena on signals received by the mobile terminal.
In one preferred embodiment of the invention, the said first antenna(s) is (are) also used in transmission.
In one preferred embodiment of the invention, the said means of combining received signals comprise:
for each distinct polarization, Viterbi decoding means outputting firstly the most probable signals, and secondly a probability factor associated with the said most probable signals;
digital signal processing (DSP) means comprising:
for each distinct polarization, weighting means weighting the most probable signals for each distinct polarization by the associated probability factor;
means of summating the signals output from the various weighting means.
This type of Viterbi decoding means determines the most probable information transported by signals received by the mobile terminal on each of the reception channels. The Viterbi decoding means that use the Viterbi algorithm are well known to the expert in the subject, and therefore are not described here.
However, it is obvious that an expert in the subject would know how to use other decoding types without going outside the framework of this invention.
Thus, the received signals on the various reception channels can be efficiently processed in base band since signals output from the first and second antennas are summated taking account of weighting factors, that are their probability factors.
According to one advantageous variant, the said means of combining the received signals comprise:
polarization selection means, that select the polarization for which the received signals have the maximum power,
Viterbi decoding means receiving signals received with the selected polarization and outputting firstly the most probable received signals, and secondly a probability factor associated with the said most probable received signals.
In this way, only the reception channel on which the highest power signals are received is preserved and processed optimally by the Viterbi decoding means.
Preferably, the said radiocommunication system belongs to the group comprising:
GSM 900 type systems;
DCS 1800 type systems;
PCS 1900 type systems;
UMTS FDD type systems;
UMTS TDD type systems;
CDMA type systems.
However, this list is not exhaustive.